This paper examines a new magnetic field assisted machining process using a magnetic machining jig. By using this process, a high efficiency and high quality internal finishing process can be achieved, and it was clarified that it was very effective to finish the internal surface of a thick tube (5~30mm in thickness). In this study, the experiments performed on the SUS304 stainless steel tubes (10mm in thickness) examine the applicability of improving the internal surface roughness and the roundness of inside tube, and it also examine the effects of a rotational speed of the magnetic machining jig. The results showed that this processing method enables improve the internal surface roughness and the roundness of inside tube. It is clarified that the roundness of inside tube is highly dependent on the rotational speed of magnetic machining jig, and the roundness can be improved rapidly at a higher rotational speed with the magnetic machining jig.